Capacitive micromachined acoustic transducers have become more and more popular in today's sensor world. The efforts, however, have been mainly focused on the making of micromachined acoustic transducers to sense the passing acoustic signal. In other words, they are mainly used as microphones. There are also some efforts to develop the micromachined ultrasonic transducers which operate in the MHz or even higher frequency ranges. For example, U.S. Pat. Nos. 5,619,476; 5,870,351; 5,894,452, and 6,493,288 incorporated herein by reference, describe the fabrication of capacitive-type ultrasonic transducers in which membranes are supported above a substrate by insulative supports such as silicon nitride, silicon oxide and polyamide. The supports engage the edges of each membrane. A voltage applied between the substrate and a conductive film on the surface of the membrane causes the membrane to vibrate and emit sound waves.
Referring to FIG. 1, traditional micromachined capacitive ultrasonic transducers are made up of multiple small sealed, evacuated cells, each including a membrane 1 coated with a metal electrode 2. The membrane 1 is supported at its edges spaced from conductive base 3 by an insulating support 4. The interior volume 5 is evacuated. The geometry and the material of the membrane, and the surrounding medium determine the mechanical response of the transducer. The CMUT cells are interconnected with metal connector 6, as shown in FIG. 2.
The CMUT transducer described in U.S. Pat. No. 6,493,288 is mainly focused on the ability to sense the acoustic pressure variations as a microphone. Devices described in U.S. Pat. Nos. 5,619,476; 5,870,351 and 5,894,452 do provide the ability of a transducer that converts electrical signal to acoustic signal, the frequency of such transducer will have to be in the ultrasonic ranges that are above 1 MHz which is not suitable for general purpose audio applications.
Capacitive acoustic actuators usually provide better frequency response than traditional magnetically driven or piezoelectric actuators. For acoustic actuator that operates in audio frequency band, it is essential that the actuator has the ability to move the surrounding air in large volume, especially in the low frequency range. Almost all the micromachined sensors and actuators focus on their ability to work in the linear range of the operation. The limit of membrane deflection in the linear range, which is usually on the order of sub-micron, makes it difficult to generate the audible sound that is large enough for human to hear.
Many efforts in building an acoustic actuator that operates in audio range were also reported in other publications and patents. In U.S. Pat. No. 6,552,469, a solid state transducer is disclosed. The transducer comprises a micromachined electrostatic actuator formed of silicon, a support brace disposed above the actuator, and a membrane coupled to the support brace. The actuator is operatively coupled to the membrane. In theory, this transducer may be either a receiver or a microphone. The issue, however, is such a device has no obvious advantages in the manufacturing and operation compared with devices made with traditional technology.